Facebook wrote some code that was so faulty that it revealed to terrorists the names of FB moderators who were charged with shutting down terrorist postings. As the Guardian reported:

Of the 1,000 affected workers, around 40 worked in a counter-terrorism unit based at Facebook’s European headquarters in Dublin, Ireland. Six of those were assessed to be “high priority” victims of the mistake after Facebook concluded their personal profiles were likely viewed by potential terrorists. The Guardian spoke to one of the six, who did not wish to be named out of concern for his and his family’s safety. The Iraqi-born Irish citizen, who is in his early twenties, fled Ireland and went into hiding after discovering that seven individuals associated with a suspected terrorist group he banned from Facebook – an Egypt-based group that backed Hamas and, he said, had members who were Islamic State sympathizers – had viewed his personal profile … Overwhelmed with fear that he could face retaliation, the moderator, who first came to Ireland as an asylum seeker when he was a child, quit his job and moved to eastern Europe for five months.

Any complex system can have issues, but, once discovered,

the bug in the software was not fixed for another two weeks … By that point the glitch had been active for a month. However, the bug was also retroactively exposing the personal profiles of moderators who had censored accounts as far back as August 2016.